Electric blanket control



May 30, 1950 A. E. RUDAHL ELECTRIC BLANKET CONTROL Filed Jan. 17, 1947arwa/nio b ARA/OLD E. PUD/4 HL Patented May 30, 1950 UNITED STATESPATENT OFFICE ELECTRIC BLANKET CONTROL Application J afruary 17, 1947,Serial No. 722,57 6

18 Claims. 1

The present invention relates to electric blankets, and moreparticularly to control circuits therefor whereby the heat generated inthe blanket may be closely regulated so as to avoid undesirabletemperature variations within the blanket and to control heat lossestherethrough within relatively narrow limits.

- It is an object of the invention to provide a control circuit forelectric blankets which is more highly sensitive than controlsheretofore utilized so as to maintain the temperature of the blanketwithin a close range under all conditions of operation.

It is a further object of the invention to provide a control circuit forelectric blankets which is sensitive to temperatures within the blanketitself and also to ambient temperature, if desired, thereby assuring theclose control of the temperature, as referred to above, and alsoproviding a safety factor in that any abnormal heating of the blanket isimmediately detected and corrected thereby eliminating the hazards offire.

A further object of the invention is to provide a control circuit whichis reliable and efficient during use, and which can be used with safetyparticularly insofar as heat will not be generated within the blanketunless the control circuit is functioning properly.

Other objects will be in part obvious and in part pointed out more indetail hereinafter.

The invention accordingl consists in the features of construction,combination of elements and arrangement of parts which will beexemplifled in the construction hereafter set forth and the scope of theapplication of which will be indicated in the appended claims.

In the drawing the single figure is a wiring diagram of a specificembodiment of the heating and control circuits of an electric blanketarranged in accordance with the invention.

Referring to the drawing, the reference numeral I denotes a blanket inwhich is arranged a heating element The blanket It may be formed of anyconventional blanketing material and the heating element may be in theform of a thin, flexible resistance wire suitably coated with a thinlayer of insulating material which is disposed throughout the blanket ina plurality of convolutions whereby heat is supplied substantiallyuniformly throughout the blanket. This construction of an electricblanket is conventional and well known in the art and therefore need notbe described in more detail here.

The resistance element l2, which also is disposed within the blanketIII, is an element whose resistance varies substantially in relationshipto its temperature. Preferably, the resistance element I2 is one whoseresistance decreases in substantially equal increments for each degreeof temperature decrease of the element, such resistances being wellknown in the art. The resistance element l2 may be formed of aninsulated thin pliable resistance wire or may be in the form of a seriesof resistance buttons, or both. Whatever form of resistance element isutilized, the same is preferably distributed throughout the blanket soas to be affected by the temperature conditions throughout the blanket.

The heating element II is connected directly to the power lines l3 and Mwhich, for example, may be of an ordinary household lighting circuit,through a thermal relay IE to be described more full hereinafter, anddoes not operate through the transformer l6 which is provided merely tofurnish the electrical energy required to operate the control circuitand therefore may be quite small. This transformer l6 has its primaryconnected to the power lines I 3 and It while the secondary of thetransformer is tapped at a suitable number of points denoted by thereference numerals I1, [8, I9, 2|, 22 and 23 to provide the variousvoltages required in the control circuit.

Taps I9, 2| and 22 are selected so that the voltages between taps I9 and2| and taps 2| and 22, respectively, are substantially in the sameproportion as the resistance ratio to one another of resistance elements24 and I2. These voltages are applied to a bridge circuit whichcomprises the said portions of the secondary between the taps l9 and 2|and taps 2| and 22, respectively, a fixed resistance 24 having a zero ornegligible temperature coeiiicient of resistance, hereinafter referredto as the reference resistance, and the blanket resistance element I2.In series with the blanket resistance element l2 there is shown a secondresistance element 25 which is optional and may be included if it isdesired to have the control circuit responsive to ambient temperature.The resistance element 25 is adapted to function in a simi lar manner tothat of resistance l2, i. e., it is so constructed that its resistancechanges in relationship to changes in its temperature. The resistanceelement 25 is external to the blanket I0 and may be located in thecontrol box, for example whereby it is affected only by roomtemperatures as distinct from temperatures within the blanket therebyadding a component into the control circuit which is sensitive toambient temperatures. If desired, however, the resistance element 25 maybe omitted and the blanket resistance element I2 may be directlyconnected to the tap 22.

The triode 2! is utilized for simplicity of presentation to illustrate atype of vacuum tube amplifier, but this. has been done merely forconvenience. If desired, other types of electronic tubes, such asamplifier or relay tubes, or a plurality of tubes, could be utilized aswill be apparent to one skilled in the art. The heater voltage for thefilament 28 is obtained from across the taps IT and I8. The cathode 29is connected to. the tap 2i while the tap 23 is connected to the plate3% through a heater wire 38 of the relay I5. The grid 3| is connectedthrough a limiting resistance 32 to the terminal 39 of the bridgewhereby it is biased to control current flow through the tube 27responsive to the degree of balance or unbalance of the bridge.

The thermal relay I comprises a pair of bimetallic switch arms 33 and 34having contact points 35 and 33 respectively. Under normal conditions,that is, at ordinary room temperature, the contact points 35 and 36 areseparated. However, the bi-metallic switch arms 33 and 34 are soarranged that, when switch arm 34 is heated, as, for example, by meansof the heater wire 38, the heated bi-metallic switch arm will bendinwardly so as to close the contacts 35 and 36. On the contrary, whenswitch arm 33 is heated as, for example, by means of the heater wire 31,that arm will bend outwardly thus tending to separate the contact points35 and 36. The heater wire 38 is connected in series between the plate30 and the transformer tap 23 as previously mentioned. The heater wire3? is connected through the thermal relay I5 with the power lines I3 andI l.

The operation of the above described embodiment of the invention will bereadily understood from the above description taken in connection withthe following explanation. Beginning with the circuits in the conditionshown in the drawing and assuming that the blanket IE is in a relativelycold condition, the total resistance of resistance I2 and resistance 25,if the latter resistance is included, is chosen to be such that thevoltage, if any, impressed on the grid 3-I of the tube 21 by the bridgecircuit will permit the fiow of current through the tube 27 and thusthrough the heater wire 38. The flow of current through the heater wire38 causes the bi-metallic switch arm 3d to become heated and thus closethe contact points 35 and 36. Immediately upon this happening, theheating circuit to the resistance element II is closed and heat is thensupplied to the blanket. The closing of this heating, circuit alsocloses the circuit through the heater wire 31. Accordingly, thebi-metallic switch arm 33 will become heated and will eventually openthe thermal relay I5 thereby opening the circuit through the heaterelement I I and the heater wire 31. The bi-metallic switch arm 33 isthen permitted to cool, thereby returning the contact point 35 towardthe contact point 33. If the blanket is still in a cold condition, acurrent will be passing through the heater wire 38, as previouslydescribed, thereby flexing the bi-metallic switch arm 34 toward thebi-metallic switch arm 33 so that, upon cooling of the bi-metallicswitch arm 33, the thermal relay I5. will again close and energize theheater element I I. If, on the other hand, the blanket is now at adesired higher temperature, the resulting change in resistance ofresistance element I2 causes a change in the degree of balance orunbalance of the bridge circuit which is sufficient by reason of thechange in grid voltage to cause a reduced current or no current to fiowthrough the tube 27 and hence through the heater wire 38, therebypermitting contact point 36 to move further away from contact point 35so that the thermal relay I5 will not close when the bi-metallic switcharm 33 cools. The use of the bridge circuit not only provides greatersensitivity in detecting changes in the resistance of the controlresistances I2 and 25, but also assures that fluctuations in linevoltage will be of no consequence.

In efiect, the thermal relay I5, by reason of the heater wire 31,functions as a simple cycling relay for opening and closing the contacts35 and 36 periodically. However, by reason of the second bi-metallic arm34 under control of the control circuit, the relative duration of theoff and on periods is controlled by the temperature within the blanket.The greater the difference between the resistance I2 and referenceresistance 26, and therefore the colder the blanket, the nearer will becontact 36 to contact 35, thus causing the heating periods tobe ofgreater duration. This arrangement is highly sensitive because of theinfinite variations in the setting of: the thermal relay I5 in responseto temperature conditions with the result that the temperature withinthe blanket I3 is maintained at substantially a constant or at leastwithout being subject to any wide fluctuations in temperature.

By inclusion of the resistance 25., the control circuit may be maderesponsive to both the temperature within the blanket and to ambienttemperature. If the resistance isomitted, the control circuit, ofcourse, is sensitive only to temperatures within the blanket itself.

The arrangement is effective for overcoming the hazards of fire becauseof the sensitivity of the control circuit to temperatures within theblanket. A further safety factor lies in the fact that, if there is anyfailure of the control circuit, such as by failure of the tube 21 or thelike, the thermal relay IE will not close and therefore the system failssafe.

There is thus provided, in accordance with the invention, a controlcircuit for electric blankets which is highly sensitive in operation soas to provide close control of the heat generated within the blanket,which circuit is safe and reliable in operation and is not likely tomalfunction or re quire frequent replacement or repair.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language used in the followingclaims is intended to cover all of the generic and specific features ofthe invention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween.

I claim as my invention:

1. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element whose resistance increases inrelationship to increases in its temperature, circuit meansfor-connecting the heating element to a source of electrical energy, apair of cooperating contact points in said circuit adapted to open andclose the circuit, electrically operated means for moving one of saidpoints in a closing direction, means for supplying said electricallyoperated means with an electric current of a magnitude proportionate todecreases in the resistance of said resistance element, electricallyoperated means for actuating the other contact point in an openingdirection, and means for supplying an electric current to said meansresponsive to closing of the contact points.

2. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistanceincreases in relationship to increases in its temperature, a circuit forconnecting the heating element to a source of electrical energy, aswitch arm normally opening said circuit and movable in one directionfor closing said circuit, electrically operated means for causing saidarm to move in said one direction, means responsive to changes in theresistance of said resistance element for supplying an electric currentto said electrically operated means when the resistance of theresistance element falls below a predetermined minimum, a second armcooperating with the first arm and movable in the opposite direction foropening said circuit to terminate the heating period, electricallyoperated means for actuating the second arm, nd means for supplying anelectric current to the last named means responsive to closing of theswitch arms.

3. A heat control arrangement for an electrica1ly heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket and subject totemperatures within the blanket, a second resistance element exterior tosaid blanket and subject to ambient temperatures, said resistanceelements exhibiting resistance changes in relationship to changes intheir temperatures, a circuit for connecting the heating element to asource of electrical energy, means movable in one direction for closingsaid circuit to initiate a heating period, electrically energized meansfor actuating said movable means in said one direction, means responsiveto changes in the resistance of said resistance elements for energizingthe actuating means with an electric current which varies in magnitudein relationship to changes in the resistance of said resistanceelements, means cooperating with the first named movable means andmovable in the opposite direction for opening said circuit to terminatethe heating period, electrically energized means for actuating the lastnamed movable means in said opposite direction, and means for energizingthe last named means responsive to closing of the circuit.

4. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistanceincreases in relationship to increases in its temperature, circuit meansfor connecting the heating element to a source of electrical energy aswitch having a bimetallic switch arm constructed and arranged to opensaid circuit when heated, means energized by closing of the switch forapplying heat to said arm, and a second bimetallic switch armconstructed and arranged to close said circuit when heated, and meansfor applying heat to said arm responsive to decreases in the resistanceof said resistance element below a predetermined minimum.

5. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a circuit for connecting the heating element to a sourceof electrical energy comprising a normally open thermal relay having apair of cooperating bimetallic switch arms one of which when heated isoperable to close the relay and circuit and the second of which armswhen heated is operable to open the relay and circuit, a resistanceelement in said blanket whose resistance changes in relationship tochanges in its temperature, means for heating the first switch arm,means for energizing the last named means at varying rates in proportionto the resistance of said resistance element, means for heating thesecond switch arm, and means for energizing the last named meansconditioned for operation by closing of the relay.

6. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, resistance element whose resistance increases inrelationship to increases in its temperature, a heating circuitcomprising said heating element and a normally open thermal relay havingcooperating bimetallic switch arms one of which when heated is operableto close the circuit and the second of which arms when heated isoperable to open the circuit, electrical heating means for the second ofsaid switch arms, means for energizing the said heating means connectedthereto through the relay only when said relay is closed, electricalheating means for the first of said switch arms, and means for applyingelectrical energy to said last named heating means responsive to changesin resistance of said resistance element.

7. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, resistance element in said blanket whose resistanceincreases in relationship to increases in its temperature, a normallyopen thermal relay having cooperating bimetallic switch arms one ofwhich when heated is operable to close the switch and the second ofwhich arms when heated is operable to open the switch, electricalheating means for each of said switch arms, circuit means for connectingthe heating element to a source of electrical energy through said switcharms, means for energizing the heating means of the second switch armactuated in response to the closing of said circuit means by the switcharms, and means for energizing the heating means of the first switch arminversely in relationship to the resistance of said resistance element.

8. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistanceincreases in relationship to increases in its temperature, a heatingcircuit for connecting the heating element to a source of electricalenergy, means for closing and opening said circuit including a pair ofnormally spaced apart contact means, electrically energized means formoving one of said contact points in a circuit closing direction andelectrically energized means for moving the other of said contact pointsin a circuit opening direction, a bridge circuit including saidresistance element having a balance af- 7 fectedby the change ofresistance of said element, means for energizing the first namedelectrically energizedmeans responsive to the balance of said bridgecircuit, and means for energizing the last named electrically energizedmeans responsive to closing of the contact points.

9 A. heat control arrangement for an electricall'y heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket subject totemperatures within the blanket, a secondresistance element exterior tothe blanket subj'ect to ambient temperatures, said resistanceelementsexhibiting resistance changes in relationship to changes: intheir temperatures, a normally open thermal relay having a. pair ofbimetallic switch arms, one of said arms when heated being. adapted tomove in a relay closing direction and the second of said arms whenheated being adapted to move in a relay opening direction, a heater wirefor each of said arms, a circuit including said relay for connecting theheating element to a sourceof electrical energy, means for energizingthe heater wire of said second switch arm of the relay responsive toclosing of the relay, and means for energizing the heater wire of thefirst switch arm of the relay proportionately to decreases in resistanceof said resistance elements.

10; A heat. control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistancechanges in relationship to changes in its temperature, a normally openheating circuit for connecting the heating element.

to a source of electrical energy, a bridge circuit including saidresistance element for producing a voltage drop substantially inproportion to the change in resistance of said resistance element,electrically operated means for closing said heating circuit, amplifyingmeans responsiveto said voltage drop for energizing the last namedmeans,electrically operated means operative for opening said heating-circuit,and means connected to the heating circuit for energizing the last namedelectrically operated: means responsive to: closing of the heatingcircuit.

11. A heat control. arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistancechanges in relationship to changes in itstemperature, a normally openthermal relay having a pair of cooperatingbimetallic switch: arms, oneof said arms when heated being adapted to move in a relay closingdirection and the second'oi said arms when heated being adapted to movein a relay opening direction, a heater wire for each of said switcharms, a circuit. including: said relay for connecting the heatingelement and the heater Wire of the second switch arm to asource. ofelectrical energy, said circuitbeing closed byclosing of the relay, andamplifying means for energizing the heater Wire of the first switch, armof the relay responsive to changes in resistance of said resistanceelement.

12:. A heat control arrangement for an electrically heated blanketcomprising, in. combination, a blanket, a resistance element in saidblanket whose resistance changes in relationship to, changes in itstemperature, a normally open thermalrelay having; a; pair of cooperatingbimetallic switch arms, one. of said: arms when heated being adapted tomove in a; relay closing direction (ill and the second of said arms whenheatedbeing adapted to move in a relay opening direction, a heater Wirefor each of said switch arms, a circuit including said relay forconnecting the heating element and the heater Wire of the second switcharm to a source of electrical energy, said circuit being closed byclosing of the relay, a bridge circuit including said resistance elementadapted to be balanced at a predetermined temperature of said element,and means for energizing the heater wire of the first switch arm of therelay in relationship to the degree of unbalance of said bridge circuit.

13. A heat control. arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket where it is subjectto temperatures within the blanket, a, second resistance elementexternal to the blanket where it is subject to ambient temperatures,said resistance elements exhibiting resistance changes in relationshipto changes in their temperature, a normally open thermal relay having apair of cooperating bimetallic switch,

-- arms, one of said arms when heated being adapted to move in a relayclosing direction and. the secondof said arms when heated being adaptedto more in a relay opening direction, a heater wire for each of saidswitch arms, a circuit including said relay forconnecting the heatingelement and the heater wire of the second switch arm to a source ofelectrical energy, said circuit being closed by closing of the relay, abridge circuit for producing a voltage drop in relationship to thechange in resistance of said resistance elements, and amplifying meansfor energizing the heater wire of the first switch arm in relationshipto changes in said voltage drop.

14. A. heat control arrangement for an electrically heated blanketcomprising, in combination, a. blanket, an electrical heating element insaid blanket, a. resistance element in said blanket whose resistancechanges in relationship to changes irritstemperature, a normally openthermal: relay having a pair of cooperating bimetallic:

heater wire for each of said switch arms, a cirsupplying electricalenergy to the heater Wire of the first switch, arm from the secondary ofsaid transformer controlled by said bridge circuit.

15. A heat control arrangement for an electrisally heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element. in said blanket. Where it is;subject to temperatures within the blanket, a second resistance elementexterior to said blanket where it is subject. to ambient temperatures,said resistance elements exhibiting re-* sistance changes inrelationship to changes. in their temperature, av normally open thermal.relay having a pair of cooperating bimetallic switch ar-ms ,.one of saidarms when heated being: adapted to move in a relay closing direction andthe second of said arms when heated being adapted to move in a relayopening direction, a heater wire for each of said switch arms, a circuitincluding said relay for connecting the heating element and the heaterwire of the second switch arm to a source of electrical energy, saidcircuit being closed by closing of the relay, a transformer adapted tobe connected to the source of electrical energy, a bridge circuitcomprising, in sequence, two sections of the secondary of saidtransformer, said resistance elements and a reference resistance havinga negligible temperature coefficient of resistance, and amplifying meansfor energizing the heater wire of the first switch arm in relationshipto the voltage drop across said bridge circuit.

16. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistancechanges in relationship to changes in its temperature, a normally openthermal relay having a pair of cooperating bimetallic switch arms, oneof said arms when heated being adapted to move in a relay closingdirection and the second of said arms when heated being adapted to movein a relay opening direction, a heater wire for each of said switcharms, a circuit including said relay for connecting the heating elementand the heater Wire of the second switch arm to a source of electricalenergy, said circuit being closed by closing of the relay, a transformeradapted to be connected to the source of electrical energy, a bridgecircuit comprising two sections of the secondary of said transformer,said resistance element and a reference resistance having a negligibletemperature coefficient of resistance, and amplifying means forsupplying electrical energy to the heater wire of the first switch armin relationship to the unbalance of said bridge circuit.

17 A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistancechanges in relationship to changes in its temperature, a normally openthermal relay having a pair of cooperating bimetallic switch arms, oneof said arms when heated being adapted to move in a relay closingdirection and the second of said arms when heated being adapted to movein a relay opening direction, a heater wire for each of said switcharms, a circuit including said relay for connecting the heating elementand the heater wire of the second switch arm to a source of electricalenergy, said circuit being closed by closing of the relay, a transformeradapted to be connected to the source of electrical energy, a bridgecircuit comprising two sections of the secondary of said transformer,said resistance element and a reference resistance having a negligibletemperature coefiicient of resistance, and a circuit for energizing theheater wire of the first switch arm comprising the secondary of saidtransformer and a vacuum tube having a grid voltage determined by thedegree of balance of said bridge circuit.

18. A heat control arrangement for an electrically heated blanketcomprising, in combination, a blanket, an electrical heating element insaid blanket, a resistance element in said blanket whose resistancechanges in relationship to changes in its temperature, a normally openthermal relay having a pair of cooperating bimetallic switch arms, oneof said arms when heated being adapted to move in a relay closingdirection and the second of said arms when heated being adapted to movein a relay opening direction, a heater wire for each of said switcharms, a circuit including said relay for connecting the heating elementand the heater wire of the second switch arm to a source of electricalenergy, said circuit being closed by closing of the relay, transformeradapted to be connected to the source of electrical energy, a bridgecircuit comprising two sections of the secondary of said transformer,said resistance element and a reference resistance having a negligibletemperature coefficient of resistance, a circuit for energizing theheater wire of the first switch arm comprising the secondary of saidtransformer and a vacuum tube having a grid voltage determined by thebalance of said bridge circuit, said bridge circuit being designed toprovide a grid bias on said tube sufficient to prevent flow of currentwhile said resistance element is above a predetermined temperature.

ARNOLD E. RUDAHL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,276,589 Steinmetz Aug. 20, 19181,694,264 Hull Dec. 4, 1928 2,302,603 Davis et a1 Nov. 17, 1942 FOREIGNPATENTS Number Country Date 144,669 Great Britain Sept. 9, 1921 338,880Great Britain Nov. 18, 1930

